[libvirt] Proposal PCI/PCIe device placement on PAPR guests

[David Gibson]

There was a discussion back in November on the qemu list which spilled
onto the libvirt list about how to add support for PCIe devices to
POWER VMs, specifically 'pseries' machine type PAPR guests.

Here's a more concrete proposal for how to handle part of this in
future from the libvirt side.  Strictly speaking what I'm suggesting
here isn't intrinsically linked to PCIe: it will make adding PCIe
support sanely easier, as well as having a number of advantages for
both PCIe and plain-PCI devices on PAPR guests.

Background:

 * Currently the pseries machine type only supports vanilla PCI
   buses.
    * This is a qemu limitation, not something inherent - PAPR guests
      running under PowerVM (the IBM hypervisor) can use passthrough
      PCIe devices (PowerVM doesn't emulate devices though).
    * In fact the way PCI access is para-virtalized in PAPR makes the
      usual distinctions between PCI and PCIe largely disappear
 * Presentation of PCIe devices to PAPR guests is unusual
    * Unlike x86 - and other "bare metal" platforms, root ports are
      not made visible to the guest. i.e. all devices (typically)
      appear as though they were integrated devices on x86
    * In terms of topology all devices will appear in a way similar to
      a vanilla PCI bus, even PCIe devices
       * However PCIe extended config space is accessible
    * This means libvirt's usual placement of PCIe devices is not
      suitable for PAPR guests
 * PAPR has its own hotplug mechanism
    * This is used instead of standard PCIe hotplug
    * This mechanism works for both PCIe and vanilla-PCI devices
    * This can hotplug/unplug devices even without a root port P2P
      bridge between it and the root "bus
 * Multiple independent host bridges are routine on PAPR
    * Unlike PC (where all host bridges have multiplexed access to
      configuration space) PCI host bridges (PHBs) are truly
      independent for PAPR guests (disjoint MMIO regions in system
      address space)
    * PowerVM typically presents a separate PHB to the guest for each
      host slot passed through

The Proposal:

I suggest that libvirt implement a new default algorithm for placing
(i.e. assigning addresses to) both PCI and PCIe devices for (only)
PAPR guests.

The short summary is that by default it should assign each device to a
separate vPHB, creating vPHBs as necessary.

  * For passthrough sometimes a group of host devices can't be safely
    isolated from each other - this is known as a (host) Partitionable
    Endpoint (PE).  In this case, if any device in the PE is passed
    through to a guest, the whole PE must be passed through to the
    same vPHB in the guest.  From the guest POV, each vPHB has exactly
    one (guest) PE.
  * To allow for hotplugged devices, libvirt should also add a number
    of additional, empty vPHBs (the PAPR spec allows for hotplug of
    PHBs, but this is not yet implemented in qemu).  When hotplugging
    a new device (or PE) libvirt should locate a vPHB which doesn't
    currently contain anything.
  * libvirt should only (automatically) add PHBs - never root ports or
    other PCI to PCI bridges

In order to handle migration, the vPHBs will need to be represented in
the domain XML, which will also allow the user to override this
topology if they want.

Advantages:

There are still some details I need to figure out w.r.t. handling PCIe
devices (on both the qemu and libvirt sides).  However the fact that
PAPR guests don't typically see PCIe root ports means that the normal
libvirt PCIe allocation scheme won't work.  This scheme has several
advantages with or without support for PCIe devices:

 * Better performance for 32-bit devices

With multiple devices on a single vPHB they all must share a (fairly
small) 32-bit DMA/IOMMU window.  With separate PHBs they each have a
separate window.  PAPR guests have an always-on guest visible IOMMU.

 * Better EEH handling for passthrough devices

EEH is an IBM hardware-assisted mechanism for isolating and safely
resetting devices experiencing hardware faults so they don't bring
down other devices or the system at large.  It's roughly similar to
PCIe AER in concept, but has a different IBM specific interface, and
works on both PCI and PCIe devices.

Currently the kernel interfaces for handling EEH events on passthrough
devices will only work if there is a single (host) iommu group in the
vfio container.  While lifting that restriction would be nice, it's
quite difficult to do so (it requires keeping state synchronized
between multiple host groups).  That also means that an EEH error on
one device could stop another device where that isn't required by the
actual hardware.

The unit of EEH isolation is a PE (Partitionable Endpoint) and
currently there is only one guest PE per vPHB.  Changing this might
also be possible, but is again quite complex and may result in
confusing and/or broken distinctions between groups for EEH isolation
and IOMMU isolation purposes.

Placing separate host groups in separate vPHBs sidesteps these
problems.

 * Guest NUMA node assignment of devices

PAPR does not (and can't reasonably) use the pxb device.  Instead to
allocate devices to different guest NUMA nodes they should be placed
on different vPHBs.  Placing them on different PHBs by default allows
NUMA node to be assigned to those PHBs in a straightforward manner.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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